the sign of \((rd^{\prime }-r^{\prime }d) \tilde{W}_\tau (\gamma ,\gamma ^{\prime })\)  is positive for \(\mu <\mu _1\)  and negative for \(\mu >\mu _1\) . Thus, the flow crosses from the side of the wall where a two-particle bound state of charges \(\gamma \)  and \(\gamma -\gamma ^{\prime }\)  is stable, into the side where the bound state decays.
In particular, it follows from (REF ) that the flow cannot cross the same wall more than once,
and that BPS states do not decay as one follows the attractor flow in reverse [1]}.
